System of distribution



May 26, 1931. A. H. MITTAG 1,307,426

SYSTEM OF DISTRIBUTION Filed Jan. 24, 1925 Fig. I.

To Load Fig.2.

VOLTS OUTPUT K W OUTPUT Patented May 26, 1931 UNITED STATES PATENT OFFIGE ALBERT H. MITTAG, F SCHENECTADY, NEW YORK, ASSIGNOR TO GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK SYSTEM OF DISTRIBUTION Application filed January 24, 1925. Serial No. 4,612.

My present invention relates to a system of electrical distribution in which it is desired to change direct current of one value into alternating current of another value, and more particularly to the use in such a system of electron discharge devices for effecting the conversion of the current. In the present case a mercury vapor discharge device having a controlling grid interposed between an anode and the cathode is employed. Such devices have a very low voltage drop and have the further characteristic that the plate current can be prevented from starting by means of the grid, but when once started cannot be stopped by means of the grid.

In transforming direct to alternating current by means of a mercury vapor device of the type described, it is necessary in order to commutate the current to connect a capacity across either the primary winding connected with the source of power or the secondary winding connected to the load. I have found it preferable, however, to connect the capacity across the primary winding. \Vith such an arrangement it has been found that, for best results, the value of the com mutating capacity should vary with the load transmitted by the apparatus. If the ca pa city employed is too large the counter electromotive force of the main transformer, under light load conditions, will rise to such a value that the discharge device will fail to hold back the current after commutation has been completed and a short circuit condition will result. On the other hand if the capacity is too small the condenser will fail to completely commutate the plate current under full load conditions, and again a short circuit condition will result.

In order, therefore, that the current through the discharge device may be properly commutated and short circuit conditions avoided, it is apparent that the capacity associated with the discharge device should be varied in such a manner that it is propor-.

tional to the load supplied by the system, and

it is an object of my invention to provide means for automatically affecting such variation.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention, however, both as to its organization and its method of operation will best be understood with reference to the following specification taken in connection with the accompanying drawings in which Fig. 1 represents a circuit or anization whereby my invention may be carried into effect, and Fig. 2 represents a series of curves showing the relation between the output voltage and the kilowatt output when different capacities are used in connection with the tube.

I have indicated in Fig. 1 an electron discharge device comprising a main bulb 1 having at one end a cathode 2 of mercury and auxiliary anodes 3 and i also of mercury on either side of the cathode 2. An arc may be constantly maintained between the cathode 2 and anodes 3 and 4L by means of an alternating source of potential 5. The bulb l is supplied with sidearms within which are located working anodes 6 and 7 and controlling grids 8 and 9. Current is supplied from mains 10 and 11 through the discharge device to the primary winding 12 of the transformer. The secondary winding 13 of the transformer is connected to a load not shown. Direct current main 10 is connected to the middle point 14 of winding 12 and main 11 is connected to the cathode element of the discharge device so that current impulses supplied by the latter flow in opposite directions through winding 12.

The discharge device comprises in effect two valves. Each valve is controlled by means of voltages applied to its grid. The connections for this purpose comprise a transformer 15, the primary Winding of which is connected across the load circuit through a condenser 16. The terminals of the secondary winding of transformer 15 are connected to the grids 8 and 9 through resistances l7 and 18. The mid point of the secondary winding is connected to the cathode through a source of energy 19 which is arranged to give a negative bias to each grid.

A condenser 20 is connected across winding 12. In order, however, that the capacity connected across winding 12 may vary with the load, a transformer 21 is associated with the output circuit, so that the current induced in the secondary winding of the transformer will be proportional to the load current. A series of relays 22 to 25 inclusive are connected to the terminals of the secondary winding of transformer 21 and are arranged to be operated in response to certain current values. Each relay is arranged to operate in response to a different load on the alternating current circuit and to control an associated switch, 26, 27 28 or 29, which in turn connects a condenser 30, 31, 32 or 33, across the primary winding 12 soas to vary the capacity in shunt to the winding 12 as required for proper operation of the apparatus.

In explaining the operation of the above described apparatus, it will be assumed that the valve including the anode 6 is rendered conductive first. Current will now flow through the left hand portion of the winding 12, inducing animpulse in the secondary winding 13 and in the right hand portion of the winding 12. The condenser 20 will become charged to the full potential of the winding 12, which is approximately twice that of the direct current circuit. At the same time the potential of the winding 13 is applied to the circuit including the transformer 15 and condenser 16. During the initial portion of this impulse, the potential will appear principally across the transformer 15 and the connections are such that a negative potential is impressed upon the grid 8 associated with the anode 7 to prevent the starting of an arc to that anode. t some later point in this impulse, the potential across the transformer 15 is reduced in value or actually reversed in polarity so that the potential applied to the grid 8 increases positively to permit an arc to start to the anodeZ. The capacitor 20 is now short circuited through the discharge device and tends to send a reverse current to the anode 6, thus instantly interrupting the current to this anode. efore capacitor 20 has become completely discharged, the valve path including the anode 6 is deionized and the grid 9 becomes charged to a negative potential, thus preventing the restarting of the arc to the anode 6. Current now flows from the direct current circuit through the right hand portion of the winding 12, inducing an impulse of opposite polarity in the winding 13. In this manner the load current is successively transferred between the anodes 6 and 7 and alternating current is supplied to the load by the winding 13.

In case the load current is reduced to a small value and a large capacity is connected to the winding 12, all of the direct current which is required to successively charge the commutating capacitor to opposite polarities at the frequency determined by the grid circuit must'appear as energy stored in the circuit elements principally the smoothing reactor and the commutating capacitor 20. The result is that the voltage in the circuit will build up to such a value that the input equals the losses in the apparatus at this extreme voltage. However, by reducing the capacity across the winding 12 until it is substantially in resonance with the magnetizing reactance of this winding, the charging current of the capacitor is substantially neutralized by the magnetizing current of the winding and the apparatus will not oscillate to continuously higher voltages.

On the other hand, when the apparatus is delivering a substantial load two efiects occur which combine to require a larger commutating capacity. In the first place, during the commutating period the full direct current is forced through the commutating capacitor in such a direction as to charge it to an opposite polarity so that, obviously, the period required to reverse the polarity of the capacitor will vary directly with the magnitude of the direct current. This is so because of the fact that the smoothing reactor tends to maintain the direct current constant; the capacitor cannot change its potential instantaneously; the potential of the windings of the output transformer vary directly with the terminal potential of the capacitor, since they are connected in parallel, and thus the direction of the flow of load current cannot reverse until after the polarity of the commutating capacitor 20 has reversed. That is, the

greater the magnitude of the direct current,

the morequickly will the commutating capacitor 2O reverse polarity and the more quickly must the discharge device be deionized in order that the grid may regain control. In the second place, the amount of ionization of the discharge device. increases with the magnitude of the current transmitted therethrough so that a longer deionizationperiod is required when the device is carrying larger current. Thus, for most satisfactory operation the commutating capacity should be in creased with increases in load. The result of such increases is shown by the curves of Fig. 2.

I have found that in the operation of mercury vapor discharge devices of the character described the best results are obtained if the grids aremade positive for less than half the time. This may be accomplished by giving a negative bias to the grids by means of the source of energy 19. In this way the grids will have an appreciable negative potential when current ceases to flow and will thus he more effective in preventing the current from starting than would be the case if their potential was'zero or only slightly negativ-e. While I have shown but one modifica-' tion of my invention, it is apparent that the invention is not limited to the particular em bodiment set forth. Other forms will suggest themselves to persons skilled in the art without departing from the scope of my invention as set forth in the appended claims.

WVhat I claim as new and desire to secure by Letters Patent in the United States is l. The combination in a system of distribution of a source of direct current, a primary winding connected to said source through a plurality of electric valves, a secondary winding inductively associated with said primary Winding, a plurality of capacity units adapted to be connected to one of said windings, a load circuit supplied by said secondary winding, and means responsive to an electrical condition in the load circuit for varying the capacity connected to one of said windings.

2. The combination in a system of distribu tion of a source of direct current, a primary winding connected to said source through an electric valve, a plurality of capacity units adapted to be connected to the primary winding, a secondary winding inductively associated with said primary winding, a load circuit supplied thereby, and means for varying the capacity connected to the primary winding in accordance with an electrical condition in the load circuit.

3. The combination in a system of distribution of a source of current, a primary winding connected to said source through an electric valve, a plurality of capacity units associated with the primary winding and adapted to be connected thereto, a secondary Winding inductively associated with said primary winding, a load circuit supplied thereby, and means controlled by the current in said load circuit for controlling the connections between said capacity units and said primary winding.

4. The combination in a system of electrical distribution, of a source of direct current, a primary winding connected to said source through a vapor electric device, provided with a grid for controlling the transmission of current between its cathode and anode, a secondary winding inductively associated with the primary winding, capacity means arranged to be interconnected with one of said windings for commutating the current transmitted through said vapor electric device, and means controlled in accordance with an electrical condition of one of said windings for regulating the effect of said capacity means.

In witness whereof, I have hereunto set my hand this 23rd day of January, 1925.

ALBERT H. MITTAG. 

